1. Field of the Invention
The present invention relates to a video-on-demand system for the supply of audio-visual programs via a cable network in accordance with user requests from a number of users and the method for the management of image data used therein.
2. Description of the Related Art
Recent years have witnessed the spread of CATV systems in a number of countries, with the principle format for the supply of programs being a pay-by-view system wherein the user can watch programs, such as movies or whatever, which are broadcast at a scheduled time. There have also been advances in the possibilities for two-way communication between the user and the cable network which broadcasts the program, so that there have been experiments using systems wherein the user can manipulate the program broadcast on a cable network using the same functions (rewind, fast forward, slow motion) which would be available if he/she were watching the program reproduced by a VCR. However, such techniques are still at a developmental stage, so that the functions present for VCRs have not yet been realized for CATV systems, and for the time being, an improved system for gathering information about users' desired viewing and a control method for such a system are necessary.
The following is an explanation of a video-on-demand system constructed according to present techniques, and the image data control method used therein, with reference to the drawings.
FIG. 1 shows a construction of a video-on-demand system according to the prior art. This video-on-demand system includes an image data transmission unit 111 for transmitting image data to the user, an image data storage unit 112 for storing the image data, usually composed of magnetic discs 113, and an image data management unit 114 for managing the storage information for the image data storage unit 112 and the information about the users, and is connected via a cable network 116 to the users' terminals 115.
FIGS. 2A and 2B show the image management tables for managing the image data in the image data management unit 114 in the video-on-demand system constructed according to the prior art, which was shown in FIG. 1.
These image management tables are comprised of a title table 121 and a frame table 122. The title table 121 stores all of the titles of the programs stored in the image data storage unit 112 together with the first address of the frame information for each title in the frame table 122.
The programs (image data) in the image data storage unit 112 are managed in a format divided into areas of a fixed length.
The frame table 122 stores for every frame the storage position (address) in the image data storage unit 112 of the first address of the image data (frames) divided into areas of fixed length. For example, the program indicated by the title information 123 for title A (program A) in the title table 121 is divided into A1, A2, A3, A4, . . . A7200 as shown in the frame information 124 for program A in the frame table 122.
On receiving a request of the title of the program which a user wishes to view from the terminal 115, the image data transmission unit 111 requests the appropriate title information in the title table 121 from the image data management unit 114. Having obtained the necessary title information, the image data transmission unit 111 requests from the image data management unit 114 the frame information corresponding to the title of the program, for example, frame information 124 for program A, out of the frame table 122, by reading the first address for the frame information 124. Having obtained the necessary frame information 124 out of the frame table 122, the image data transmission unit 11 stores this frame information 124, and accesses the address corresponding to the title of the program in the image data storage unit 112, thereby requesting the image data. Having obtained the image data, it then transmits the image data via the cable network 116 to the user's terminal 115.
The following is an explanation of the operation of the video-on-demand system constructed as described above.
FIG. 3 is a drawing showing the transmission process for the image data for the video-on-demand system of the prior art shown in FIG. 1, including the interaction with the user. The areas enclosed by broken lines show the operations performed by the image data transmission unit 111, the image data storage unit 112, the image data management unit 114, and the user's terminal 115, respectively.
The user accesses the image data transmission unit 111 using the terminal 115 via the cable network 116, and requests the program to be viewed, for example, program A (step 1302). The image data transmission unit 111 then requests the title information 123 for program A in the title table 121 from the image data management unit 114 (S1304). The image data management unit 114 then transmits the program A title information 123 in the title table 121 to the image data transmission unit 111 (S1306), when the program requested by the user is A. The image data transmission unit 111 obtains this program A title information 123 and accesses the address in given in the program A title information 123 from the image data management unit 114, before requesting the frame information 124 which corresponds to program A in the frame table 122 (S1308). The image data management unit 114 then transmits the requested frame information 124 (S1310). The image data transmission unit 111 obtains this frame information 124 from the image data management unit 114, and stores this frame information 124 (S1312). The image data transmission unit 111 then successively accesses the addresses on the magnetic disc 113 in the image data storage unit 112 given in frames A1 through A7200 in said frame information 124 (S1314). The image data storage unit 112 then transmits the accessed image data (S1316). In this way, the image data transmission unit 111 obtains the necessary image data from the image data storage unit 112 (S1318). The image data transmission unit 111 then transmits the obtained image data to the user's terminal 115 (S1320). The transmitted image data is then televised using the terminal 115 (S1322). The user watches the televised image, and may make use of the same operations (rewind, play, fast forward) as with a VCR (S1324). The image data transmission unit 111 then judges whether such a VCR operation has been made (S1326) and, if there has been such an operation, receives the operation, returns to step S1314, and has the image televised according to the wishes of the user (S1314-S1322). If there has not been a VCR operation, then the image data transmission unit 111 checks whether the final frame has already been transmitted (S1328), repeating the process in steps S1314-S1326 when the program has not finished, and moving on to step S1330 when it has. Having transmitted all of the image data, the image data transmission unit 111 then executes a post-completion process (S1330), for example, a fee-charging process.
For video-on-demand systems under the prior art, there have been no problems so long as programs are sent to the users at a fixed speed, but, when using such program supplying techniques, if there are a number of users watching the same program at the same time, all watching the same part of the program repeatedly by rewinding and fast forwarding, then there is the possibility that the system will be unable to send the image data to the user uninterruptedly. The same problems may occur if there are a number of titles stored on the same recording medium.
The situation described below can be thought of as a typical example. When magnetic discs 113 are installed in the image data storage unit 112, then if the average retrieval rate is 2 MBytes/second and the necessary transfer rate for transmitting image data to 1 user is set at 256 KBytes/second, then this means that a total of around 8 users can view images from one magnetic disc 113. If 10 magnetic discs 113 are arranged in parallel, then, if the load is shared effectively, a total of 8.times.10=80 users can be served. Therefore, if the requesting of a title by the users is performed at a predetermined fixed interval, then the video-on-demand system can commence the transmission of the image data. However, if after a given time, all 80 users make use of operations such as fast forward or rewind which all concern the images stored on 1 disc out of the magnetic discs 113, then, since the image data storage unit 112 can only transfer the data for 8 users, there will be a breakdown in the transmission of image data to the users' terminals 115, and the normal transmission of image data will no longer be possible.
Also, since it is necessary for the image data transmission unit 111 to store all of the frame information for the programs requested by the users, then it is necessary to equip the image data transmission unit 111 with a large storage capacity.